In the field of an image forming apparatus using an electrophotographic image forming process, a process cartridge type of apparatus unifies a photosensitive member and process means actable on the photosensitive member, such as charging means, developing means, cleaning means or the like) into a cartridge which is detachably mountable to a main assembly of the image forming apparatus. The process cartridge type of apparatus is advantageous in that maintenance of the apparatus can be carried out by the user of the apparatus without a service person, so that operability is remarkably improved. Therefore, the process cartridge type of apparatus is widely used in the field of the image forming apparatus. It is desirable that the process cartridge is mounted to the main assembly of the image forming apparatus with high positional accuracy between the process cartridge and the main assembly of the image forming apparatus, while providing a proper mounting feeling.
In a widely used system for correctly positioning a cartridge relative to the main assembly of the image forming apparatus, a projection is provided on the cartridge, and correspondingly, the main assembly of the apparatus is provided with a positioning portion for engagement with the projection, and when the projection is in engagement with the positioning portion, the cartridge is urged by a spring to assure the support, as disclosed in Japanese Laid-open Patent Application Hei 11-174940.
In another example, as shown in FIG. 9, the drive transmission structure is different.
An axial end of a photosensitive drum 67 is provided with a twisted polygonal prism projection 616, and correspondingly, a coupling end 617 of main assembly side of the image forming apparatus which transmits a driving force to the photosensitive drum 67, is provided with a twisted hole having a polygonal cross-section. When the projection 616 is engaged with the hole, they are rotatable integrally with each other so that a rotational driving force is transmitted, during which a thrust force is produced to pull the projection into the main assembly side of the apparatus by the twisting configuration, and in addition, the axis of the photosensitive drum 67 is positively aligned with the axis of the drum gear 622 by way of the coupling, thus determining the position of the cartridge correctly relative to the main assembly of the apparatus, as disclosed in Japanese Laid-open Patent Application Hei 10-104905.
However, the conventional image forming apparatus involves a problem in the operability in the positioning and removal of the cartridge.
In the spring latch type apparatus disclosed in Japanese Laid-open Patent Application Hei 11-174940, the spring for positioning the cartridge provides a resistance against the falling of the cartridge in the main assembly of the apparatus. If the spring force is reduced, the resistance is reduced, but the positioning force is also reduced, and therefore, doing so is not desirable. As a result, the cartridge may rise, or ride on another element, so that the cartridge might not be positioned correctly. This results in an excessive load applied to the photosensitive drum and to the transfer belt unit disposed opposed thereto, and may lead to damage in the belt within an image forming region thereof, and then, the image formation is adversely influenced.
The structure of Japanese Laid-open Patent Application Hei 10-104905 will be considered. In order to smoothly remove the cartridge, the coupling desirably has already been retracted. To accomplish this, a rotation of the twisted hole is required to disengage from the projection 616, and therefore, a rotation link mechanism is necessary which is interrelated with some operation, with the result of a complicated structure, and therefore, an increase in the number of parts, in the cost and in the limitation of the arrangements of parts.